The European standards relating to diesel fuel pollution, in particular those applicable to heavy goods vehicles, have made it possible to bring influence to bear on an effective and significant reduction in emissions which are harmful to the environment. Firstly, with phases EURO2 and EURO3, the combustion parameters of these engines for heavy goods vehicles have been modified. Phase EURO 4 has resulted in the manufacturers of heavy goods vehicle engines choosing after-treatment for exhaust gases, i.e. SCR (Selective Catalytic Reducer), EGR (Exhaust Gas Recirculation) and particle filters (PF). These various after-treatments can be installed on their own or in combination, inasmuch as that they do not always act on the same pollutants present in the exhaust gases.
The majority of European manufacturers of heavy goods vehicle engines have opted for SCR after-treatment for their engines' exhausts, this after-treatment acting exclusively on the reduction of nitrogen oxides present in the gases, another advantage of this technique being that it allows a significant reduction in fuel consumption by optimum engine adjustments. The SCR after-treatment consists of reducing the NOx or nitrogen oxides on a catalyst containing platinum and palladium, and in the presence of gaseous ammonia. In order to introduce gaseous ammonia into the exhaust, it is known to produce it directly in the pipe before the SCR by vaporizing an aqueous urea solution which, placed at an average temperature generally fluctuating from 200 to 400° C., decomposes progressively to gaseous ammonia.
In certain installation configurations for the SCR and for the injection of urea, the manufacturers have noted the appearance of deposits in exhaust pipes before the inlet to the SCR. These deposits can be sufficiently large to give rise to a partial or even total blockage of the exhaust pipe, thus causing a loss of engine power. At a constant injection configuration, the quantity of deposits formed is greater at low temperatures than at high temperatures. According to the analyses which have been carried out, the major part of these deposits is constituted of cyanuric acid, resulting from incomplete decomposition of the urea. This cyanuric acid can sublimate and produce gaseous ammonia again; this reaction can only take place at a very high temperature, greater than 450° C., a temperature rarely reached at this location in exhaust pipes.
Moreover, it has been noted that these deposits were present in pipes having bends due to lack of space in the vehicle, and where the distance separating the injection of urea and the first bend is too short. The assumption is that in this type of configuration, a part of the urea droplets does not have time to vaporize and fully decompose to gaseous ammonia. They are deposited on the wall of the pipe, which is at a temperature too “cold” to allow complete decomposition to gaseous ammonia, and they decompose only partially, forming deposits of cyanuric acid which adhere to the wall.
U.S. Pat. No. 5,489,419 describes a process for reducing the pollutants generated during combustion, more particularly NOx, by selective non-catalytic reduction (SNCR), using a Nox-reducing agent, generally urea, in aqueous solution. In order to increase the reliability of introducing the urea solution into the combustion chamber, D1 proposes to add a surfactant to the aqueous solution of urea in order to obtain a spray of urea in which the average size of the droplets is reduced and their distribution is more restricted than a spray of urea without surfactant. U.S. Pat. No. 5,645,756 proposes to improve the reliability of apparatus in contact with aqueous solutions of urea, prepared with water having a hardness which is not negligible, due to the presence of calcium, magnesium, carbonate ions etc. (cf. col. 3, lines 13-49). To solve this technical problem, D2 proposes to add to the aqueous solutions of urea a compound in order to suppress the hardness of the water, chosen from water-soluble phosphonates.
WO 00/75643 describes a method for monitoring the quantity of a urea solution used in a system intended to reduce the NOx, in particular in a combustion process with selective catalytic reduction (SCR). This method consists of incorporating a fluorescent tracer in the urea solution and monitoring the development the signal as a function of time, this fluorescent tracer moreover being capable of having surfactant properties. No document of the prior art mentions the formation of deposits, apart from particular documents which propose new injector structures for these solutions in order to avoid such the formation of deposits, without specifying their nature.